P. Kozyra

T–O–T skeletal vibration in CuZSM-5 zeolite: IR study and quantum chemical modeling

The location of Cu cations in CuZSM-5, properties of cationic sites and their interaction with guest molecules have been studied by quantum chemical (DFT) modeling and IR spectroscopy based on the frequency shift of antisymmetric T–O–T vibration of oxygen rings. The shift has been found sensitive both to the framework interaction with cations and to the interaction with adsorbed molecules. It has been measured and estimated theoretically from parameters characterising framework distorsion by Cu+ and Cu2+, with MgZSM-5 and NaZSM-5 used as ‘‘reference samples’’. It was found that the ordering of the cation perturbing effect was: Na+<Cu+<Mg2+<Cu2+. NO interaction with Cu cations was much stronger than that of CO and N2 . Divalent copper showed polarized 2-electron covalent bonding with NO strengthening its bond while moderate bonding ability of monovalent copper led to NO bond activation, in accordance with high catalytic activity of Cu+ZSM-5.

Why Cu+ in ZSM-5 framework is active in DeNOx reaction—quantum chemical calculations and IR studies

Abstract In this paper, we present quantum chemical calculations for copper sites in the extended model of ZSM-5 framework based on seven T sites in two fused 5T rings cut off the MFI structure. Geometrical and electronic properties of cationic copper centres and their adsorption complexes with NO molecule are analysed from the point of view of the activation mechanism. The calculations show that the NO molecule adsorbed on Cu+ site becomes significantly activated contrary to Cu2+ centre, in full accordance to IR measurements. Our results indicate clearly that the ability of Cu+ site to donate electrons to π-antibonding orbital of NO causes significant bond weakening and makes Cu+ZSM-5 active in decomposition of nitrogen oxides.

On the nature of spin- and orbital-resolved Cu+–NO charge transfer in the gas phase and at Cu(I) sites in zeolites

Electronic factors essential for NO activation by Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels (natural orbitals for chemical valence). NOCV analysis is performed for three DFT-optimized models of Cu(I)–NO site in ZSM-5: [CuNO]+, (T1)CuNO, and (M7)CuNO. NO as a non-innocent, open-shell ligand reveals significant differences between independent deformation density components for α and β spins. Four distinct components are identified: (i) unpaired electron donation from NO π‖* antibonding orbital to Cus,d; (ii) backdonation from copper dyz to π⊥* antibonding orbital; (iii) donation from occupied π‖ and Cu dxz to bonding region, and (iv) donation from nitrogen lone-pair to Cus,d. Channel (i), corresponding to one-electron bond, shows-up solely for spin majority and is effective only in the interaction of NO with naked Cu+. Channel (ii) dominates for models b and c: it strongly activates NO bond by populating antibonding π* orbital and weakens the N–O bond in contrast to channel (i), depopulating the antibonding orbital and strengthening N–O bond. This picture perfectly agrees with IR experiment: interaction with naked Cu+ imposes small blue-shift of NO stretching frequency while it becomes strongly red-shifted for Cu(I) site in ZSM-5 due to enhanced backdonation.

Properties of sodium ions in zeolite materials: FT-IR study of the low temperature adsorption of carbon monoxide

The low temperature adsorption of CO has been investigated on NaX (Si/Al a. r. = 1.3), NaY (Si/Al a. r. = 2.4), NaMOR (Si/Al a. r. = 6.5) and NaFER (Si/Al a. r. = 25) zeolites by FT-IR spectroscopy. For comparison CO adsorption has also been investigated on Na-silica–alumina. Bands assigned to CO C-bonded and O-bonded to Na+ ions have been observed and discussed. A correlation between the position of the band and the siting and coordination state of Na+ ions has been observed for C-bonded species. Some effect of the cavity has been envisaged for O-bonded species. Low frequency bands corresponding to quite stable species have also been found and assigned to carbon monoxide bonded to two cations. This suggests that CO may also probe couples of the nearest Na+ ions, that may act in adsorption.

Speciation of cobalt in CoZSM-5 upon thermal treatment

Transformation of cobalt centres in CoZSM-5 of various (Co/AlO 4 ) ratios was investigated as a function of the thermal treatment in the temperature range of 290–830 K, by means of IR, UV-Vis-NIR, TG-QMS, and EPR spectroscopies, supported by DFT calculations. The focus was devoted to elucidation of the speciation of cobalt inside and outside the channels of the ZSM-5 zeolite. A molecular description of the principal phenomena (dehydration, hydrolysis, olation, and oxolation) involved in the thermal activation was provided and rationalised in terms of partial charge model.

The activation of c=c bond in alkenes by cu+ ions in zeolites ir, tpd-ir studies and dft calculations

Abstract IR studies evidenced that Cu + ions in zeolites were able to activate C=C bond in but-1-ene. The stretching frequency of C=C- bond decreased by about 100 cm 1 upon adsorption of but-1-ene on Cu + in CuX and CuZSM-5. The bands of CH stretching and deformation in =CH 2 fragment did also shift upon the adsorption. The adsorption of but-1-ene was so strong, that it removed CO and NO preadsorbed on Cu + ions. The experiments TPD-IR showed the heterogeneity of Cu + sites in CuX and the presence of at least two kinds of Cu + of various energies of but-1-ene bonding. Quantumchemical DFT calculation revealed a distinct weakening of C=C bond and evidenced that it was caused by a π-back donation of d-electrons of Cu + to antibonding π* orbitals of C=C- in but-1-ene.

Heterogeneity of Cu+ in CuZSM-5, TPD-IR studies of CO desorption

TPD-IR experiments of CO desorption were performed. The process of CO adsorption was followed by IR spectroscopy and the process of desorption was studied both by TPD and by IR. TPD-IR studies evidenced that three kinds of Cu+ sites of increasing energy of CO adsorption were present in CuZSM-5. They are characterised by IR bands of adsorbed CO at 2165, 2160 and 2155 cm−1 respectively. The same three kinds of Cu+ sites were found also in NaCuZSM-5 zeolites of lower Cu contents (Na/Cu exchange degrees 20 and 40%). If small amounts of CO is adsorbed at room temperature, CO adsorbs on all three kinds of Cu+ species without selecting the most favourable ones. At higher temperatures (340–460 K) CO desorbs from less favourable sites of higher CO stretching frequencies (2165 and 2160 cm−1) and readsorbs on most favourable ones (CO band 2155 cm−1). The sites of the lowest C-O stretching frequency (2155 cm−1) show the strongest electrodonor properties and the strongest effect of π-backdonation of d-electrons of copper to π* antibonding molecular orbital of CO. This effect weakens the C-O bonding. Cu+ sites of the lowest C-O frequency are probably the most effective in activation of NO molecule and therefore the most active in “denox” process.

Cu+, Ag+ and Na+ Cationic Sites in ZSM-5 Interacting with Benzene: DFT Modeling

Abstract The present work is focused on the effect of zeolite framework on the activation of benzene by Cu+, Ag+ and Na+ ions. DFT calculations have been carried out to obtain geometric structure and electronic properties of both the cluster models of the cationic sites and bare cations interacting with benzene. NOCV (natural orbitals for chemical valence) has been used to elucidate components of differential electron density: donation and back donation. Zeolite framework is shown to intensify the benzene activation for several reasons, e.g. imposing specific adsorption geometry or modifying cation properties.

DFT and IR studies on copper sites in CuZSM-5: structure—redox conditions—denox activity relationship

In this work we have studied by quantum chemical (DFT) modelling electronic and geometrical properties of copper centres in β position in ZSM-5. The results were compared with our earlier data concerning α sites. It was evidenced that the properties of both Cu 2+ and Cu + in α sites differ from those in β sites. This is the most pronounced for Cu + the positive charge of which is to the largest extent neutralised by framework oxygens when located in α sites. Consequently Cu + in α site has the highest energy of HOMO orbital, it is therefore the best electron donor to π * antibonding orbital of NO and activates the most the adsorbed molecule.

Extinction coefficients of CC and CC bands in ethyne and ethene molecules interacting with Cu+ and Ag+ in zeolites--IR studies and quantumchemical DFT calculations.

The values of extinction coefficients of CC and CC IR bands of ethyne and ethene interacting with Cu+ and Ag+ in zeolites were determined in quantitative IR experiments and also by quantumchemical DFT calculations with QM/MM method. Both experimental and calculated values were in very good agreement validating the reliability of calculations. The values of extinction coefficients of ethyne and ethene interacting with bare cations and cations embedded in zeolite-like clusters were calculated. The interaction of organic molecules with Cu+ and Ag+ in zeolites ZSM-5 and especially charge transfers between molecule, cation and zeolite framework was also discussed in relation to the values of extinction coefficients.